Hydrocarbon purification process

ABSTRACT

In the removal of HF from hydrocarbon streams by chemical reaction with solid potassium hydroxide, runaway temperatures and subsequent explosions can occur when excess HF is present in the hydrocarbon charged to the KOH treater. According to the invention, a control system and method are provided which regulate the flow of hydrocarbon charged to the KOH treater responsive to temperature changes within the system indicative of excessive amount of HF in the hydrocarbon stream, thereby preventing the treater temperature from exceeding a preselected maximum allowable value. Several embodiments are provided whereby temperature changes in the KOH treater or a differential temperature across an HF stripper are used as temperature sensing points to control flow of hydrocarbon to the KOH treater.

This invention relates to the removal of HF from hydrocarbon streamscontaining same. In accordance with one aspect, this invention relatesto the continuous separation of hydrofluoric acid from a hydrocarbonstream employing a solid potassium hydroxide treating agent and thecontrol of separation whereby excessive runaway temperatures are avoidedin the KOH treater. In another aspect, this invention relates to amethod and apparatus for controlling the purification of a hydrocarbonstream containing HF by contacting with KOH wherein the flow ofhydrocarbon charged to the KOH treater is manipulated in response to apreselected maximum allowable temperature within the KOH treating zone.In accordance with another aspect, this invention relates to a controlmethod and apparatus for purifying a hydrocarbon stream containing HF bycontact with KOH wherein the flow of hydrocarbon charged to the KOHtreater is manipulated in response to a differential temperature betweenan intermediate point in an HF stripper and temperature of the effluentstripped material removed from the stripper so that the temperature inthe KOH treater does not exceed a preselected maximum allowabletemperature.

In a process for the conversion of hydrocarbons wherein liquid hydrogenfluoride (HF) is employed as a catalyst, small amounts of HF acid andorganic fluorides are present in the product streams due to thesolubility of these materials in hydrocarbons. In most commercialoperations, the hydrocarbon phase containing organic fluorides and HF isrecontacted with relatively pure liquid HF to remove the organicfluorides therefrom, as described in U.S. Pat. No. 3,254,137, issued May31, 1966, Hutto et al. In some operations, the propane and normal butaneyields are treated with a solid reagent such as bauxite or alumina toremove organic fluorides therefrom as described in U.S. Pat. No.3,527,840, issued Sept. 8, 1970, to Price. With substantially all of theorganic fluorides removed, the propane and the normal butane separateyields can be treated with solid KOH to remove the remaining HF, asdescribed in the above Hutto et al patent. When organic fluorides arenot first removed, as above described, then the propane and normalbutane yields are each separately treated with solid KOH in the presenceof added alcohol as described in U.S. Pat. No. 3,403,198, issued Sept.24, 1968, to VanPool. This organic fluoride removal, using bauxite oralumina, is effected between the HF stripper and the solid lump KOHtreater. KOH removes substantially only HF from the hydrocarbon.

It is necessary to remove the HF from these streams before subsequentprocessing or blending of the hydrocarbon streams. Normally, the amountof HF present in the hydrocarbon stream is relatively small but the HFstill has to be removed from the hydrocarbon streams in order that thehydrocarbons will pass the fluoride specification for the respectivestreams. Residual amounts of HF are ordinarily removed by contact withsolid KOH. Upsets often occur in the processing equipment, therebycausing excess HF in the stream to be charged to the solid bed of KOHparticles. When too much HF contacts the KOH, the temperature starts torise in the bed area due to the heat of reaction between KOH and HF. Ifexcess HF is allowed to continue to flow to the KOH treater, runawaytemperatures are experienced which can cause hydrocarbons charged tovaporize and "blow up" the KOH treater with danger then of fires, etc.The present invention is directed to an improved system of controllingthe flow of hydrocarbon streams containing HF to a KOH treater in orderto prevent runaway temperatures and subsequent explosions.

Accordingly, an object of this invention is to provide an improvedmethod and control system for the removal of HF from hydrocarbonstreams.

A further object of this invention is to provide a temperature-sensitivecontrol system for regulating the flow of hydrocarbon streams containingHF to a treater in a practical and economical manner.

A further object of this invention is to provide a sensitive and rapidresponse control system and method for the purification of hydrocarbonstreams.

Other objects, aspects, and the several advantages of this inventionwill be apparent to those skilled in the art upon a study of thedisclosure, the drawings, and the appended claims.

In accordance with the invention, a method and a control system areprovided for manipulating the flow of hydrocarbon containing HF to a KOHtreater whereby the flow of hydrocarbon is regulated responsive totemperatures in the separation system in such a manner that thetemperature in the KOH treater does not exceed a preselected maximumallowable value.

In accordance with one embodiment, excessive temperature increases andsubsequent explosion in the KOH treater used to remove HF from ahydrocarbon stream, e.g., liquid propane, are avoided by utilizing apreselected maximum differential temperature across the reboiler of anHF stripper to manipulate the control of flow of bottoms charged to theKOH treater or to a by-pass around the treater.

A better understanding of the invention will be obtained upon referenceto the accompanying drawings wherein:

FIG. 1 is a schematic flow diagram of an alkylation process and oneembodiment of the invention;

FIG. 2 is a schematic flow diagram of another embodiment of theinvention; and

FIGS. 3 and 4 are alternative embodiments for controlling the flow ofhydrocarbon feed to a KOH treater.

In FIG. 1 an alkylation system is illustrated comprising a reactor orcontact zone 10 having inlet conduits 11 for olefin such as propyleneand/or butylenes, 12 for isoparaffin such as isobutane, and 13 formakeup and rerun hydrogen fluoride (HF) catalyst. Effluent from contactzone 10 is removed via conduit 14 and passed to phase separator orsettler 15 wherein the HF phase settles and is removed for recycle viaconduit 16 to contact zone 10 with a portion being charged to a rerunsystem (not shown) for removal of impurities.

The hydrocarbon phase is removed from separation zone 15 by way of line17 and passed to fractionation zone 18 (which can be a plurality ofdistillation columns) wherein HF is removed by line 19, alkylate isremoved by line 20, normal butane (vapor) is removed by line 21, andisobutane is removed by way of line 22 and recycled (not shown) tocontact zone 10.

Propane is often present in the fresh isobutane feed, with propylenefeed, and some propane is produced in the process. In order to prevent abuildup of propane in the system, the stream is passed from conduit 17by way of line 23 to depropanizer 24. Isobutane and heavier is passed byway of line 25 to conduit 17 and then to fractionator 18. Overheadproduct comprising propane, HF, and alkyl fluorides (when present) ispassed by way of line 26 and condenser 27 to phase separator 28. LiquidHF accumulates in sump or leg 29 and is withdrawn by way of line 30.Hydrocarbon liquid, principally propane, containing dissolved HF andalkyl fluoride, e.g., isopropyl fluoride, when present, is withdrawn byway of line 31. A portion of the condensate in 28 is passed by way ofline 32 as reflux to depropanizer 24.

The yield portion of the hydrocarbon liquid removed from settler 28 byline 31 is passed by way of line 33 to HF stripper 34 for removal of HF.Overhead product comprising HF and propane passes via conduit 35 andcondenser 27 back to phase separator or accumulator 28. Bottoms productcomprising propane containing a small or trace amount of HF and, if notpreviously removed, containing alkyl fluorides (in which case there is atreatment thereof with such as bauxite or alumina upstream, not shown,of the KOH treater) is passed by way of conduits 36 and 37 to contactvessel 38 containing a bed of solid KOH. Propane of very substantiallyreduced HF content is removed as product by way of line 39. A slurry ofwater, KOH, and KOH--HF reaction product (slough) is removed from KOHtreater 38 by way of line 40 for disposal or for recovery of KOH. Thepropane product stream removed overhead from treater 38 by way of line39 is substantially dry and free of HF.

Stripper 34 is operated under conditions sufficient to take overheadmost of the HF present in the feed, together with some hydrocarbon, andas bottoms a hydrocarbon stream substantially freed of HF. Stripper 34can be heated indirectly by steam or other heating medium in a lowerportion of the stripper, preferably by indirect heat exchange. In actualoperation, for the stripping of HF from a propane stream, thetemperature in the upper portion of stripper 34 is ordinarily in therange of about 105° F to about 140° F (40°-60° C), and the bottomtemperature is ordinarily about 120° F to about 155° F (49°-61° C). Thepressure existing in stripper 34 is ordinarily about 250 psig (1,725 kPag.) to about 350 psig (2,415 kPa g.).

The rate of withdrawal of bottoms product (which has been heatedindirectly with steam) removed from stripper 34 by way of line 36 iscontrolled responsive to liquid level controller 41 which manipulatesthe position of valve 42 in accordance with the desired level of liquidin the bottom of stripper 34. The temperature of the bottoms streamremoved from stripper 34 is usually somewhat higher than desired forcontact with KOH in treater 38, and, accordingly, is cooled to atemperature of about 100° F (37.8° C) in heat exchanger 43. The flow ofheat exchange fluid through heat exchanger 43 is controlled bytemperature controller 44 which regulates the position of valve 45responsive to the temperature sensed in line 36 downstream of heatexchanger 43. Normally, the flow of stripped hydrocarbon betweenstripper 34 and treater 38 is via lines 36 and 37 with valve 46 open andvalve 47 closed. The temperature in KOH treater 38 is sensed andtransmitted to temperature controller 49 which in turn manipulates theposition of valves 46 and 47.

In actual operation, the liquid hydrocarbon stream containing residualHF normally charged to KOH treater 38 by way of line 37 is at atemperature of about 100° F (37.8° C) and contains small amounts of HFinsufficient to cause significant increases of the temperature in theKOH treater. There is a slight warming in the KOH bed when the normallysmall amount of HF, say, about 10 to about 50 ppm, is present in thecharge introduced by line 37. When upset occurs with excess HF, as slugsof free HF, or, e.g., at least about several hundred ppm, flowing in orwith the liquid feed to treater 38, the KOH bed temperature starts toincrease rapidly. The temperature control means 49, which senses thetemperature in the KOH bed, is set for about 120°-130° F (49°-54° C).Thus, when 130° F (54° C) is reached, valve 46, which is normally open,is closed, and valve 47, which is normally closed, is opened to allowthe high HF-containing stream to be by-passed around KOH treater 38 byway of lines 48 and 55. If desired, an alarm can be sounded, say, at120°-125° F (49°-51.78° C), and the valves can be manipulated as abovebetween 125°-130° F (51.78°-54° C).

The liquid propane (at about its bubble point) removed from the bottomof stripper 34 is at about 140° F (60° C) and a pressure of 285 psig(1,970 kPa g.). The pressure in KOH treater 38 is a few pounds lowerthan in stripper 34, but at 100° F (37.8° C) inlet temperature thepropane is below its bubble point. In order to prevent vaporization ofpropane in the KOH unit, a maximum temperature to stop "flashing ofliquid to vapor" and resulting disaster is set at about 125°-130° F(51.78°-54° C) on temperature control means 49 so that only liquid willbe in treater 38.

As a further modification of the flow in FIG. 1, it is within the scopeof the invention to pass the stripped hydrocarbon stream around thetreater through alternate line 50 for return to settler 15 instead ofpassing the hydrocarbon stream through by-pass lines 48 and 55. Suitablevalves can be provided in line 55 or line 50 can replace line 55 orother modifications can be made as desired.

When pressure in conduit 39 rises to a preset maximum value, pressurecontroller 53 actuates closing of valve 54 in conduit 39.

In another embodiment of the invention, as illustrated in FIG. 2,stripper 34 and solid KOH treater 38 are positioned as described inconnection with FIG. 1. A propane stream 33 is introduced in an upperportion of stripper 34 which is operated under conditions sufficient totake overhead a vapor stream comprising HF and propane by way of line35. This overhead stream can be returned to condenser 27, as in FIG. 1,for further processing.

A bottoms stream comprising stripped propane and residual HF is removedfrom stripper 34 by line 36, passed through cooler 43 to reduce thetemperature of the stripped stream to about 100° F (38° C) forintroduction into KOH treater 38 by way of line 37. The rate ofwithdrawal of bottoms product from column 34 is controlled by liquidlevel controller 41. The rate of heat exchange fluid passed throughexchanger 43 can be controlled by temperature controller (not shown) asdescribed in FIG. 1.

In accordance with this embodiment of the invention, the temperature issensed at an intermediate point of stripper 34 and in the bottomsproduct line 36. The temperature sensed at these two points is passed toa differential temperature controller 51 which in turn controls by-passvalve 52. Under normal operating conditions, with small or trace amountsof fluorides in the stripped bottoms stream, the temperature will beabout the same at an intermediate point of stripper 34 as bottoms stream36. However, when excess HF, for example, is included with feed 33 andthe HF reaches an intermediate point of column 34, the temperature willdrop considerably at this point in stripping zone 34. A drop intemperature at an intermediate point of stripper 34 is indicative ofexcessive amounts of HF in the system, say, at least about severalhundred ppm, including even slugs of free HF therein. Differentialtemperature controller 51 is set, say, for a differential temperature of10° F to 20° F (-12.2° C to -6.67° C), and when this differential isexceeded, differential temperature controller 51 actuates valve 52 toclose feed line 37 and pass bottoms product in line 36 through by-passline 50. Valve 52 is a conventional three-way valve.

As shown in FIG. 2, when an upset is experienced with an unexpectedexcess amount of HF present in the feed to stripper 34, the flow of feedto KOH treater 38 is discontinued and the stripped hydrocarbon streamcontaining excess HF is passed around treater 38 as described in FIG. 1.Although the by-pass line is shown as line 50 for return of hydrocarbonsto settler 15, it is within the scope of the invention to by-pass thehydrocarbon to the effluent line 39 around treater 38.

The conditions obtaining in stripper 34 and treater 38 in thisembodiment are essentially as described in connection with FIG. 1.

The fractionation system disclosed in the drawing uses a separate maincolumn 18, along with the depropanizer 34-HF stripper 38 columns. Otherconventional fractionation can be used upstream of the HF stripper,e.g., as disclosed in U.S. Pat. No. 3,211,802, issued Oct. 12, 1965, toDixon et al.

In another embodiment of the invention, as illustrated in FIG. 3, solidKOH treater 38 positioned as described in connection with FIG. 1 is fedwith liquid propane containing residual amounts of HF by lines 36 and37. The hydrocarbon feed introduced into treater 38 is contacted withsolid KOH in treater 38 for removal of HF, and the effluent hydrocarbonsubstantially freed of HF is removed from treater 38 by line 39 forfurther use as desired. Normally open valve 46 is positioned in line 37,and normally closed valve 47 is positioned in line 48. Normally openvalve 60 is in the hydrocarbon outlet line, and normally closed valve 61is in line 62 passing effluent to flare as desired.

The temperature in KOH treater 38 is sensed and transmitted totemperature controller 49 which in turn manipulates the position ofvalves 46, 47, 60, and 61. In the event of excess HF in the feed passedto the treater, the temperature in the treater will increase, and if thetemperature exceeds a preset maximum, temperature controller 49 closesvalves 46 and 60 and opens valves 47 and 61 so that the feed willby-pass the treater through lines 48, 55, and 39, if desired, or throughline 50 for recycle as previously described. In addition, vaporsremaining in treater 38 can be vented through line 62.

Referring to FIG. 4, hydrocarbon feed containing HF is passed by way oflines 36 and 37 and introduced into treater 38 for contact with solidKOH to reduce HF content in the feedstream. Treated hydrocarbon isremoved by line 39 containing valve 60. Line 62 containing valve 61 isprovided for passing vapors from an upper portion of treater 38 to flareif necessary. Valves 46 and 60 are normally open, and valves 47 and 61are normally closed, as described in FIG. 3. Temperature controller 49senses the temperature at the inlet portion of the KOH bed and actuatesreversal of the valve position of valves 46, 47, 60, and 61 when thetemperature rises to above a preset value in order to prevent explosionsin the system.

We claim:
 1. In a process for removing HF acid from hydrocarbon streamscontaining same by contacting with solid KOH in a treating zone, thestep of preventing an excessive temperature increase above a preselectedmaximum allowable temperature and subsequent explosion in said treatingzone when unexpected excess HF is present in said stream charged to saidtreating zone comprising obtaining a temperature measurement in thesystem indicative of an excess amount of HF which would cause anexcessive temperature increase in said treating zone and controlling theflow of said stream passed to said treating zone in response to saidtemperature measurement by decreasing and/or terminating the flow ofsaid stream charged to said zone when the temperature measurementreaches and/or exceeds said preselected maximum value so that thetemperature within said zone does not exceed said preselected value. 2.A process according to claim 1 wherein said treating zone is providedwith a by-pass loop and the flow of said hydrocarbon stream to said zoneis controlled so that the stream charged to said zone is decreased andterminated, if necessary, as the measured temperature reaches saidpreselected maximum value and said hydrocarbon stream passed throughsaid by-pass loop so that the temperature within said zone does notexceed said preselected value.
 3. A process according to cliam 1 whereinsaid hydrocarbon stream comprises propane.
 4. In a process for removingHF acid from a hydrocarbon stream containing same by stripping thesematerials from said stream and contacting the stripped hydrocarbonstream containing residual amounts of HF with solid KOH in a treatingzone, the step of preventing an excessive temperature increase above apreselected maximum allowable temperature and subsequent explosion insaid treating zone when unexpected excess HF is present in said streamcharged to said treating zone comprising controlling the flow of saidstream passed to said treating zone in response to a temperaturemeasurement in the system indicative of an excess amount of HF whichwould cause an undesirable increase in the temperature within saidtreating zone above a preselected maximum value, said flow beingcontrolled by terminating the flow of said stream to said treating zoneand diverting said stream elsewhere so that the temperature within saidzone does not exceed said preselected value.
 5. A process according toclaim 4 comprising the steps of:measuring the temperature at anintermediate point within said stripping zone, measuring the temperatureof said stripped hydrocarbon stream, comparing said temperaturemeasurements and producing a differential temperature measurement signalrepresentative thereof, and adjusting the flow of said strippedhydrocarbon stream to said treating zone responsive to changes in saiddifferential temperature measurement signal by decreasing the flow tosaid treating zone and diverting the remainder of said strippedhydrocarbon stream to by-pass line in such a manner that the temperaturewithin the treating zone does not exceed said preselected maximum value.6. A process according to claim 4 wherein said treating zone is providedwith a by-pass loop and the flow of said hydrocarbon stream to saidtreating zone is controlled so that the stream charged to said zone isterminated as the measured temperature reaches or surpasses apreselected maximum value and the rate of flow of said hydrocarbonstream passed through said by-pass loop is commenced so that thetemperature within said zone does not exceed said preselected value. 7.A process according to claim 4 wherein said hydrocarbon stream comprisespropane and the temperature of the stream charged to said KOH treatingzone is about 100° F (38° C).